DNSOP Working Group W. Wijngaards
Internet-Draft NLnet Labs
Intended status: Standards Track November 28, 2013
Expires: June 1, 2014
Confidential DNSdraft-wijngaards-dnsop-confidentialdns-00
Abstract
This document offers opportunistic encryption to provide privacy for
DNS queries and responses.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 1, 2014.
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Internet-Draft Confidential DNS November 20131. Introduction
The privacy of the Question, Answer, Authority and Additional
sections in DNS queries and responses is protected by the
confidential DNS protocol by encrypting the contents of each section.
The goal of this change to the DNS protocol is to make large scale
monitoring more expensive, see [draft-bortzmeyer-dnsop-dns-privacy]
and [draft-koch-perpass-dns-confidentiality]. Authenticity and
integrity may be provided by DNSSEC, this protocol does not change
DNSSEC and does not offer the means to authenticate responses.
Confidential communication between any pair of DNS servers is
supported, both between iterative resolvers and authoritative servers
and between stub resolvers and recursive resolvers.
The confidential DNS protocol has minimal impact on the number of
packets involved in a typical DNS query/response exchange by
leveraging a cacheable ENCRYPT Resource Record and an optionally
cacheable shared secret. The protocol supports selectable
cryptographic suites and parameters (such as key sizes).
The client fetches an ENCRYPT RR from the server that it wants to
contact. The public key retrieved in the ENCRYPT RR is used to
encrypt a shared secret or public key that the client uses to encrypt
the sections in the DNS query and which the name server uses to
encrypt the DNS response. Note that an ENCRYPT RR must be fetched
for each name server in order for the entire session to be
confidential.
As this is opportunistic encryption, the key is (re-)fetched when the
exchange fails. If the key fetch fails or the encrypted query fails,
communication in the clear may be performed.
The server advertises which crypto suites and key lengths may be used
in the ENCRYPT RR, the client then chooses a crypto suite from this
list and includes that selection in subsequent DNS queries.
The key from the server can be cached by the client, using the TTL
specified in the ENCRYPT RR, the IP address of the server
distinguishes keys in the cache. The server may also cache shared
secrets and keys from clients.
2. ENCRYPT RR Type
The RR type for confidential DNS is ENCRYPT TBD (decimal). The
presentation format is:
. ENCRYPT [flags] [algo] [id] [data]
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The flags, algo and id are unsigned numbers in decimal and the data
is in base-64. The wireformat is: one octet flags, one octet algo,
one octet id and the remainder of the rdata is for the data. The
type is class independent and it is a hop-by-hop transaction RR type.
The domain name of the ENCRYPT record is '.' (the root label) for
hop-by-hop exchanges.
In the flags the least two bits are used as usage value. The other
flag bits MUST be ignored by receivers and sent as zeroes.
o PAD (value=0): the ENCRYPT contains padding material. Algo and id
are set to 0. Its data length is random (say 1-63 octets), and
has some random values. It is a resource record that may be
appended to resource records that are encrypted so that identical
queries encrypt to different encrypted data of different lengths.
o KEY (value=1): the ENCRYPT contains a public or symmetric key.
The algo field gives the algorithm. The id identifies the key,
this id is copied to ENCRYPT type RRS to identify which key to use
to decrypt the data. The data contains the key bits.
o RRS (value=2): encrypted data. The data contains encrypted
resource records. The data is encrypted with the selected
algorithm and key id. The data contains resource records in DNS
wireformat [RFC1034], with a domain name, type, class, ttl,
rdatalength and rdata.
o SYM (value=3): the ENCRYPT contains an encrypted symmetric key.
The contained, encrypted data is rdata of an ENCRYPT of type KEY
and has the symmetric key. The data is encrypted with the
algorithm and id indicated. The encrypted data encompasses the
flags, algo, id, data for the symmetric key.
The ENCRYPT RR type can contain keys. It uses the same format as the
DNSKEY record [RFC4034] for public keys. algo=0 is reserved for
future expansion of the algorithm number above 255. algo=1 is RSA,
the rdata determines the key size, such as 512 and 768 bits. algo=2
is AES, aes-cbc, size of the rdata determines the size of the key,
such as 128 and 192 bits.
3. Server and Client Algorithm
If a clients wants to fetch the keys for the server from the server,
it performs a query with query type ENCRYPT and query name '.' (root
label). The reply contains the ENCRYPT (or multiple if a choice is
offered) in the answer section. These ENCRYPTs have the KEY flag
set.
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Internet-Draft Confidential DNS November 2013
If a client wants to perform an encrypted query, it sends an
unencrypted outer packet, with query type ENCRYPT and query name '.'
(root label). In the additional section it includes an ENCRYPT
record of type RRS. This encrypts a number of records, the first is
a query-section style query record, and then zero or more ENCRYPTs of
type KEY that the server uses to encrypt the reply. If the client
wants to use a symmetric key, there are no ENCRYPTs of type KEY
inside the encrypted ENCRYPT data, instead an ENCRYPT of type SYM is
positioned in the outer packet, before the ENCRYPT of type RRS and
the ENCRYPT of type RRS is encrypted with the symmetric key.
If a server wants to encrypt a reply, it also uses the ENCRYPT type.
The reply looks like a normal DNS packet, i.e. it has a normal
unencrypted outer DNS packet. Because the query name and query type
have been encrypted, the outer packet has a query name of '.' and
query type of ENCRYPT and the reply has an ENCRYPT record in the
answer section with flag RRS. The reply RRs have been encrypted into
the data of the ENCRYPT record. The RR counts for every section are
stored in the outer (unencrypted) header. Thus, the combination of
the original header and the decrypted data from this record results
in the decrypted packet.
The client may lookup keys whenever it wants to. It may cache the
keys for the server, using the TTL of those ENCRYPT records. It
should also cache failures to lookup the ENCRYPT record for some time
(eg. the negative TTL if the reply contained one). Errors and also
timeouts should also be taken as an indication that the ENCRYPT
cannot be looked up, and the client MUST fall back to unencrypted
communication (this is the opportunistic encryption case). The
result of an encrypted query may also be timeouts, errors or replies
with mangled contents, in that case the client MUST fall back to
unencrypted communication (this is the opportunistic encryption
case). Note that if some middlebox removes the ENCRYPT from the
additional section of an encrypted query, likely a reply with
ENCRYPTs of type KEY is returned instead of the encrypted reply with
an ENCRYPT of type RRS, and again the client does the unencrypted
fallback. If the server has changed its keys and does not recognize
the keys in an encrypted query, it should return FORMERR, and include
its current ENCRYPTs of type KEY in that FORMERR reply. A server may
decide it does not (any longer) have the resources for encryption and
reply with SERVFAIL to encrypted queries, forcing unencrypted
fallback. Keys for unknown algorithms should be ignored by the
client, if no usable keys remain, fallback to insecure.
The client may cache the ENCRYPT of type SYM for a server together
with the symmetric secret, this is better for performance, as public-
key operations can be avoided for repeated queries. The server may
also cache the ENCRYPTs of type SYM with the decoded secret,
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associating a lookup for the rdata of the SYM record with the decoded
secret, avoiding public-key operations for repeated queries.
Key rollover is possible, use different key ids and support the old
key for its TTL, while advertising the new key, for the servers. For
clients, generate a new public or symmetric key and use it.
4. Authenticated Operation
The previous documented the opportunistic operation, where deployment
is easier, but security is weaker. This documents options for
authenticated operation. [[ is this out of scope? ]]. The client
selects if encryption is authenticated, opportunistic, or disabled.
The ENCRYPT KEYs for authority servers can be signed with DNSSEC.
The domain name of the nameserver is used to store the ENCRYPTs of
type KEY. [[ Could also be in the reverse tree for the IP address ]].
For authenticated operation, fallback to insecure should not be
performed. However, this will significantly harm deployment as
unclean lookup paths result in lookup failure. Keys with unsupported
crypto algorithms MUST still be ignored and if no keys are left,
fallback to insecure MUST still be performed, also for authenticated
operation.
The key for recursive resolvers can be configured into the stub
machines, or a domain name can be configured where the keys are
looked up and they are signed with DNSSEC.
5. Comparison with TLS and DTLS
An alternative method of accomplishing confidential DNS would be to
leverage one of the existing means for establishing a secure
transport layer. For example a secure TCP session could be
established to the name server over which DNS queries could be sent
with no changes to the DNS protocol. The most significant down side
to this approach is the burden that it places on high volume name
servers. Very large scale DNS operators expect to answer hundreds of
thousands of queries per second (possibly even more than a million
qps) for each host in their name server footprint. The use of
technologies such as IPSec or TLS may have such a severe impact on
the largest name server operators as to impede adoption of
confidential DNS.
DTLS (RFC 6347) offers a more interesting approach to securing the
connection to a name server that may be implemented in a way that is
less abusive to the large scale name servers. It looks as though the
overhead imposed by DTLS would probably be significantly higher than
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the protocol described in this draft, however if the session
established via DTLS is used over a large number of queries then the
cost of the handshake could be amortized over the total number of
queries.
6. IANA Considerations
An RR type registration for type ENCRYPT with number TBD and it
references this document [[to be done when this becomes RFC]].
7. Security Considerations
Opportunistic encryption is the main mode here. Opportunistic
encryption has many drawbacks, against active intrusion, but works
against passives. The pervasive passive surveillance problem
statement and also its security considerations are applicable to this
document. Hence the suggested short key sizes and opportunistic
encryption.
This technique does not provide perfect forward secrecy.
Additionally, timing, traffic analysis (what IP address is
contacted), packet size, RR count, header flags and header RCODE can
be observed. These could provide almost all the information that was
encrypted. Such as: query to IP address for example.com nameservers,
size of the packet is similar to a www.example.com lookup and is
followed by http packets to www.example.com's IP address.
8. Acknowledgments
Acknowledgements here
9. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
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